Full waveform inversion (“FWI”) is a method of inverting seismic data to infer earth subsurface properties that affect seismic wave propagation. Its forward modeling engine utilizes finite difference or other computational methods to model propagation of acoustic or elastic seismic waves through the earth subsurface model. FWI seeks the optimal subsurface model such that simulated seismic waveforms match field recorded seismic waveforms at receiver locations. The theory of FWI was initially developed by Tarantola (1989). Research and applications of FWI in exploration geophysics have been very active in the past decade, thanks to the dramatic increase of computing power.
It is well known that simulated waveforms depend linearly on the input source wavelet when linear acoustic or elastic wave equations are used to model seismic wave propagation. In fact, accurate estimation of source wavelet plays a critical role in FWI. Delprat-Jannaud and Lailly (2005) pointed that accurate wavelet measurements appear to be a major challenge for a sound reconstruction of impedance profiles in FWI. They noticed that a small error in the source wavelet leads to a strong disturbance in the deeper part of the inverted model due to the mismatch of multiple reflections. They concluded that “the classical approach for estimating the wavelet by minimizing the energy of the primary reflection waveform is not likely to provide the required accuracy except for very special cases”.
Indeed, inversion of primary reflections without well control faces a fundamental non-uniqueness in estimating the wavelet. For example, larger reflection events can be caused by larger impedance contrasts or a stronger source. Similar ambiguity exits for wavelet phase and power spectrum. Well data are commonly used to constrain wavelet strength and phase. But well logs are not always available, especially in an early exploration setting, or in shallow subsurface cases.
There have been extensive studies of wavelet estimation in the geophysical literature. In particular, inversion of wavelet signature for FWI was discussed by Wang et al. (2009) and the references therein. However, these methods all implicitly rely on direct arrivals or refracted waves for wavelet estimation. Because these transmitted modes propagate along mostly horizontal ray paths, they are influenced by effects (e.g. radiation pattern, complex interaction with the free surface) not affecting the near-vertical reflection ray paths. Such effects are often difficult to describe accurately and to simulate. Hence, there is a need to estimate the wavelet for the vertically-propagated energy, and this is particularly relevant for reflection-dominated applications (e.g. deep-water acquisition, imaging of deeper targets).
Multiples are considered to be noise in traditional seismic processing since they often contaminate primary reflections and make interpretation more difficult. On the other hand, it is known that multiples may also be useful for constraining subsurface properties and the seismic source wavelet. Verschuur et al. (1989, 1992) proposed a method of surface-related multiple elimination (SRME), by which wavelet estimation can be performed along with multiple elimination. The principle of SRME has been extended to an inversion scheme by G. J. A. van Groenestijn et al. (2009) to reconstruct the missing near-offset primaries and the wavelet. However it is unclear whether the optimal wavelet for SRME is also optimal for FWI. Papers such as van Groenestijn and Verschuur perform their multiple modeling and wavelet estimation in the time domain, i.e., data-driven without a subsurface model.